United States president Barack Obama says he and Prime Minister Kevin Rudd share several personality traits - one of which is humility.

The US president made the comments to the 7.30 Report's Kerry O'Brien during an interview which will be aired on ABC1 tonight.

O'Brien says during his 20-minute interview with Mr Obama, the US president shed some light on his relationship with Australia's Prime Minister.

"It was interesting. Diplomats and politicians say nice things about each other when they're having international chats," O'Brien said.

But O'Brien says Mr Obama spoke candidly about their relationship - which has in the past been described as a "meeting of minds".

"He was quite expansive and quite genuine on what he saw as the commonality and connections between [he and Mr Rudd]. One of which was humility," O'Brien said.

O'Brien discussed a range of headline issues with Mr Obama in Washington, including the war in Afghanistan.

O'Brien says Mr Obama gave more than just the political spin when discussing the bloody nine-year conflict.

"It's a mix. How many questions has Barack Obama been asked about Afghanistan since he began campaigning for the presidency? An awful lot. So there was a familiarity about the broad perspective," he told 702's Adam Spencer.

"But he engaged in areas like, when he started campaigning he talked about America having been in the war in Afghanistan for seven years. Well America has now been in the war in Afghanistan for nine years.

"I put to him that it does not seem to have got any better, and drew the obvious comparison - and I'm not suggesting it's black and white and cut and dry - but there are worrying similarities between this engagement and Vietnam.

"He took issue with that and said they have very clear objectives this time and yes they do have an exit strategy; they cannot be expected to be there forever."

O'Brien says Mr Obama was at times philosophical during the tightly controlled interview.

"I asked him how he'll measure the worth of his presidency, I asked him what it would mean for him to be a transformational president, and he says he'll have plenty time to reflect on that when he's out," he said.

The two men also discussed global warming and compared the political debates surrounding the issue in Australia and the US.

O'Brien says Mr Obama has taken a similar stance to Mr Rudd on the issue.

"He's convinced that the solution ultimately has to include some form of market-based mechanism," he said.

"He says one way or another you have to put a price on carbon."

During the interview, Mr Obama also said he was looking forward to visiting Australia in June this year.